1. Field of the Invention
The invention relates to a method of forming an upstanding collar around a hole in a sheet metal blank, which is rotated relative to a converter tool, the converter tool converting an edge region of the blank bordering said hole into the collar. The invention also relates to conversion apparatus for performing such a method comprising a converter tool.
2. Description of the Prior Art
A method of the kind described above is known in practice in the manufacture of a packing for a coil of rolled steel. This packing has two packing covers, which cover respectively the flat bottom and flat top edges of the coil of rolled steel, and a sleeve which covers the cylindrical side wall of the coil of rolled steel. In order to seal the packing against dripping water and to prevent damage occurring to the rolled steel, the packing cover is provided with a turned-up edge at its outer circumference. A method and apparatus for manufacturing a lid with a turned-up edge at the outer circumference is known from NL-A-8302807.
The packing cover can be provided with a hole corresponding with the coiler hole in the coil of rolled steel. It is desirable to provide this cover hole also with a collar around its circumference which, like the turned-up edge of the outer circumference of the packing cover has the function of sealing the packing against dripping water and preventing damage to the coil of rolled steel.
In the known method, the collar around the hole in the cover is formed by clamping the cover, already provided with its hole, so that it can rotate, with a cylindrical hollow pressure plate on one side, the cylindrical inner wall of the pressure plate having the shape of the collar required, and then, by means of spinning, gradually pushing the edge portion bordering the hole against the inner side of the cylindrical wall of the hollow pressure plate. An inconvenience of this method is that for each dimension of cover hole another size of pressure plate is needed. Not only does this represent high investment in pressure plates, but also carrying out this method is costly in that the tool has to be changed for each new dimension.
Another disadvantage is that the spinning means are costly. It is necessary to have complicated spinning means in order to prevent the consequences of elastic spring-back in the turned-back edge of the collar. Additional measures for the spinning means are required to prevent angular distortion on the transition of the face of the blank towards the collar, which make the spinning means more costly still and place high demands on its use.
Other machines suffering similar disadvantages are described in the literature.
U.S. Pat. No. 2,370,666 shows a sheet working machine for forming a flange around a hole in a sheet by clamping the sheet between rollers adjacent the hole, rotating the sheet by means of the rollers and moving a dome-shaped shaping tool in one direction parallel to the axis of rotation so as to form the flange in one step against one of the rollers. U.S. Pat. No. 2,254,289 shows a similar machine, in which a part spherical axially reciprocating shaping tool forms the flange against clamping and rotating rollers. The sheet is gradually fed past the shaping tool.
Machines in which a flange is formed around a hole by radial movement of an appropriately shaped roller, while the sheet is rotated, are disclosed in U.S. Pat. Nos. 3,709,016 and 3,924,432.
The object of the invention is to overcome the disadvantages associated with this known method.
According to the invention in one aspect there is provided a method of forming an upstanding collar around a hole in a sheet metal blank by means of a converter tool which converts an edge region of the blank bordering said hole into said collar, while the blank is rotated relative to the converter tool which method comprises the steps of
(i) in a first stage, converting a portion of said blank radially outwardly bordering said hole into a first turned-up rim,
(ii) after said first stage, in a second stage, converting a further portion of said blank radially outwardly bordering said first turned-up rim into a second turned-up rim,
(iii) in at least one further stage repeating step (ii) successively converting each time a further portion of the blank radially outwardly bordering the immediately previously turned-up rim into another turned-up rim, until the whole of said edge region has been converted,
wherein the following conditions apply:
(a) in at least one of said stages the conversion takes place on free air as herein defined, and
(b) in all said stages the conversion takes place in the same direction relative to the blank.
In the method according to the invention, a first turned-up rim is formed from a first part of the edge portion around the hole, which rim reinforces the edge portion around the hole. Then, using the reinforcement obtained, it is possible to convert a second part of the edge portion into a second turned-up rim with a larger diameter than the first turned-up rim. The step-wise conversion of the edge portion is continued until the entire edge portion has been converted and the converted edge portion forms a collar on the blank at the position of the coil hole.
This conversion of the edge portion in stages is carried out at least partly preferably entirely on free air. The term "on free air" comes from the technique of spinning. Article 2.8 on page 17 of the brochure "Forceren en Vloeidraaien van Staal", No. 301, Sixth Edition, published by "Stichting Staalcentrum Nederland", states: "One of these operations is `taking in on free air`. This is the local reduction in centreline without the presence there of a pattern form". By analogy in this application "converting on free air" is taken to mean the conversion of material without the presence there of a pressure plate or another kind of pattern form against which the material to be formed is pressed.
Preferably a part of the edge portion is converted into a turned-up rim at 90.degree. to the blank. This results in a very high stiffness of the edge portion, making the forming of a subsequent turned-up rim easier to achieve. A larger part of the edge portion to be formed can be converted at each stage.
Preferably, after a plurality of turned-up rims has been formed, the last turned-up rim already formed is converted on free air into a larger diameter turned-up rim. When this is done on all the turned-up rims formed, a collar is obtained in which the originally formed edges are eliminated and pass into each other forming a collar of the required shape. Using this feature in combination with the effect described earlier produces a collar of a circular cylindrical shape perpendicular to the plane of the blank.
As a result of the elasticity of the material of the blank, the collar springs back elastically. The spring-back is particularly noticeable at the outer end of the collar remote from the plane of the blank. Preferably at least the end part, remote from the plane of the blank, of the first turned-up rim is pushed into the size required. In this way, the consequence of the elastic spring-back is reduced to a desirable degree.
In an especially simple embodiment of the method a portion of the blank bordering on the turned-up rim last formed is pushed out of the plane of the blank within the elastic region of that portion of the blank, and the inwardly directed force generated by this on the end part remote from the blank of the first turned-up rim is opposed at least partly by the converter tool.
In practice, it appears that good results are achieved, especially with steel blanks, if the edge region is converted in stages of about 1 cm radial length.
When the method is applied in the manufacture of steel covers, good results have been achieved when the increase in height of a turned-up rim during conversion lies between 0.5 and 4 mm per rotation of the steel blank and in particular when the increase in height of a turned-up rim during conversion lies between 1 and 2 mm per rotation of the steel blank.
In another aspect, the invention provides conversion apparatus for forming an upstanding collar around a hole in a sheet metal blank by converting an edge region of the blank adjacent said hole into said collar, comprising
(a) a converter tool
(b) clamping means for said blank,
(c) means for rotating said clamping means relative to said converter tool about an axis, and
(d) means for moving said converter tool, parallel to said axis in order to perform the conversion,
(e) said converter tool comprising a mandrel which engages the metal blank during the conversion and which has in the direction of said axis a foremost end and a rearmost end, and the blank engaging surface of the mandrel being a surface of rotation which from said foremost end to said rearmost end increases in diameter non-linearly and at least partly in a plurality of steps.
With such an apparatus, especially with all increases being stepwise, a first part of the edge portion bordering on the hole may be converted while initially a second part of the edge portion bordering directly on it remains untouched. By moving the mandrel further in the direction of the blank parallel to the axis of rotation of the clamping device, the second part of the edge portion is converted.
If so desired, the mandrel can be built up from a number of separately manufactured elements, for example by taking one disc per stepwise increase of the desired diameter and stacking all the discs on top of one another. In this, the discs may be fixed relative to one another or they may be rotatable relate to one another.
In the embodiment in which individual discs are fixed relative to each other, or in which the mandrel is essentially a single solid body, the rotational speed at different positions from the foremost extremity to the rearmost extremity depends on the position. In order to reduce the speed difference between different positions on the mandrel and the parts of the edge portion already converted, preferably the magnitude of the stepwise diameter increase decreases from the foremost end towards the rearmost end. This also achieves the advantage that, the more the mandrel forms more turned-up edges, and the more it comes into contact with more material of the blank, the less quickly will the distorting forces increase with each successive conversion stage.
Preferably the increase in diameter between two successive stages is essentially nil, whereby the portion where the material of the blank is in contact with the mandrel is diminished and relative speed differences and friction have less effect.
In order to prevent heavy local distortion of the blank, the salient angles at the outwardly projecting corners of the steps of the mandrel are rounded off and in particular are rounded off at a radius of between 5 to 10 times the thickness of the blank. Practical trials have shown that with this embodiment there is a good balance between on the one hand the metallurgical reinforcing of the material of the blank as a consequence of the distortion, and on the other hand the conditions of tension occurring in the material of the blank which induce distortion.
Preferably also in a median plane two connecting lines joining the outwardly projecting corners of the steps of the rotation surface closest to the foremost end intersect each other transversely. Using such a mandrel in accordance with this embodiment, and with only a small axial movement of the mandrel, a large part of the edge portion may be converted without any large forces acting on the edge portion.
Preferably the largest diameter of the mandrel is larger than or equal to the height of the collar to be formed.
During conversion any stretching in the material occurs mainly in the circumferential direction of a turned-up rim; only a little residual distortion occurs in the height direction of a turned-up rim. That means that the radial length of the turned-up rim to be converted is approximately equal to the height of the collar formed. It is desirable that at the start of conversion, the mandrel axis should not cut the blank, because otherwise the edge of the hole displays an unstable behaviour during conversion of the first turned-up rim. Therefore, the maximum diameter of the mandrel should be preferably larger than or equal to the radial length of the whole converted edge portion. This also means that the maximum diameter of the mandrel should be preferably larger than or equal to the height of the collar to be formed.
Preferably the increase of the diameter at a base part of the mandrel adjacent the rearmost end is nil and the height of this base part is at least as large as the height of the collar to be formed. By moving the mandrel over the entire base part past the face of the blank in parallel with the axis of rotation of the clamping device, an essentially cylindrical collar is formed in the blank.
Furthermore this base part assists in setting in place the part of the collar which is already formed further from the plane of the blank, whereby that part springs back less elastically and a better shaped collar is formed. For certain uses of blanks with extended collars, it is desirable to have a collar with an even more diminished elastic spring-back, which can be achieved with the apparatus which is the mandrel is provided at its rearmost end with a pressing element for pressing up, out of the plane of the blank, a part of the blank that borders on the last formed turned-up rim. By pressing this pressing element against the transition from the flat part of the blank and the edge portion which was formed last, the transition is pressed locally and elastically out of the plane of the blank. Through this the part of the collar formed first, i.e. the upper end, undergoes an inwards force which presses a part of the upper end of the collar against the cylindrical base part of the mandrel. This distorts the upper side plastically, so that it springs elastically less far back after the mandrel has been withdrawn.
Preferably the mandrel is free to rotate, so that it is brought into rotation on its axis by the friction of the material of the blank and a separate drive for the mandrel is not needed.